Internet Engineering Task Force (IETF) B. Niven-Jenkins
Request for Comments: 6707 Velocix (Alcatel-Lucent)
Category: Informational F. Le Faucheur
ISSN: 2070-1721 Cisco
N. Bitar
Verizon
September 2012
Content Distribution Network Interconnection (CDNI) Problem Statement
Abstract
Content Delivery Networks (CDNs) provide numerous benefits for
cacheable content: reduced delivery cost, improved quality of
experience for End Users, and increased robustness of delivery. For
these reasons, they are frequently used for large-scale content
delivery. As a result, existing CDN Providers are scaling up their
infrastructure, and many Network Service Providers (NSPs) are
deploying their own CDNs. It is generally desirable that a given
content item can be delivered to an End User regardless of that End
User's location or attachment network. This is the motivation for
interconnecting standalone CDNs so they can interoperate as an open
content delivery infrastructure for the end-to-end delivery of
content from Content Service Providers (CSPs) to End Users. However,
no standards or open specifications currently exist to facilitate
such CDN Interconnection.
The goal of this document is to outline the problem area of CDN
Interconnection for the IETF CDNI (CDN Interconnection) working
group.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6707.
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RFC 6707 CDN Interconnection Problem Statement September 20121. Introduction
The volume of video and multimedia content delivered over the
Internet is rapidly increasing and expected to continue doing so in
the future. In the face of this growth, Content Delivery Networks
(CDNs) provide numerous benefits for cacheable content: reduced
delivery cost, improved quality of experience for End Users (EUs),
and increased robustness of delivery. For these reasons, CDNs are
frequently used for large-scale content delivery. As a result,
existing CDN Providers are scaling up their infrastructure, and many
Network Service Providers (NSPs) are deploying their own CDNs.
It is generally desirable that a given content item can be delivered
to an EU regardless of that EU's location or the network they are
attached to. However, a given CDN in charge of delivering a given
content may not have a footprint that expands close enough to the
EU's current location or attachment network, or may not have the
necessary resources, to realize the user experience and cost benefit
that a more distributed CDN infrastructure would allow. This is the
motivation for interconnecting standalone CDNs so that their
collective CDN footprint and resources can be leveraged for the
end-to-end delivery of content from Content Service Providers (CSPs)
to EUs. As an example, a CSP could contract with an "authoritative"
CDN Provider for the delivery of content, and that Authoritative CDN
Provider could contract with one or more downstream CDN Providers to
distribute and deliver some or all of the content on behalf of the
Authoritative CDN Provider.
A typical end-to-end content delivery scenario would then involve the
following business arrangements:
o A business arrangement between the EU and his CSP, authorizing
access by the EU to content items controlled by the CSP.
o A business arrangement between the CSP and an "authoritative" CDN
Provider where the CSP mandates that the CDN Provider perform the
content delivery on behalf of the CSP.
o A business arrangement between the Authoritative CDN Provider and
another (or other) CDN(s) where the Authoritative CDN may delegate
the actual serving of some of the content delivery requests to the
other CDN(s). A particular case is where this other CDN Provider
happens to also be the Network Service Provider providing network
access to the EU, in which case there is also a separate and
independent business relationship between the EU and the NSP for
the corresponding network access.
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RFC 6707 CDN Interconnection Problem Statement September 2012
The formation and details of any business relationships between a CSP
and a CDN Provider as well as between one CDN Provider and another
CDN Provider are out of scope of this document. However, this
document concerns itself with the fact that no standards or open
specifications currently exist to facilitate such CDN Interconnection
from a technical perspective.
One possible flow for performing an end-to-end content delivery
across a CDN Interconnection is described below:
o The initial content request from an EU's User Agent is first
received by the authoritative (upstream) CDN, which is the CDN
with a business arrangement with the CSP.
o The authoritative (upstream) CDN may serve the request itself, or
it may elect to use CDN Interconnection to redirect the request to
a Downstream CDN that is in a better position to do so (e.g., a
Downstream CDN that is "closer" to the EU).
o The EU's User Agent will "follow" the redirect returned by the
Authoritative CDN and request the content from the Downstream CDN.
If required, the Downstream CDN will acquire the requested content
from the authoritative (upstream) CDN, and if necessary the
Authoritative CDN will acquire the requested content from the
Content Service Provider.
The goal of this document is to outline the problem area of CDN
Interconnection. Section 2 discusses the use cases for CDN
Interconnection. Section 3 presents the CDNI model and problem area
being considered by the IETF. Section 4 describes each CDNI
interface individually and highlights example candidate protocols
that could be considered for reuse or leveraging to implement the
CDNI interfaces. Appendix B.2 describes the relationships between
the CDNI problem space and other relevant IETF working groups and
IRTF research groups.
1.1. Terminology
This document uses the following terms:
Authoritative CDN: A CDN that has a direct relationship with a CSP
for the distribution and delivery of that CSP's content by the
Authoritative CDN or by Downstream CDNs of the Authoritative CDN.
CDN Interconnection (CDNI): A relationship between a pair of CDNs
that enables one CDN to provide content delivery services on behalf
of another CDN. A CDN Interconnection may be wholly or partially
realized through a set of interfaces over which a pair of CDNs
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RFC 6707 CDN Interconnection Problem Statement September 2012
communicate with each other in order to achieve the delivery of
content to User Agents by Surrogates in one CDN (the Downstream CDN)
on behalf of another CDN (the Upstream CDN).
CDN Provider: The service provider who operates a CDN and offers a
service of content delivery, typically used by a Content Service
Provider or another CDN Provider. Note that a given entity may
operate in more than one role. For example, a company may
simultaneously operate as a Content Service Provider, a Network
Service Provider, and a CDN Provider.
CDNI Metadata: The subset of Content Distribution Metadata that
has an inter-CDN scope. For example, CDNI Metadata may include
geo-blocking information (i.e., information defining geographical
areas where the content is to be made available or blocked),
availability windows (i.e., information defining time windows during
which the content is to be made available or blocked) and access
control mechanisms to be enforced (e.g., URI signature validation).
CDNI Metadata may also include information about desired distribution
policy (e.g., pre-positioned vs dynamic acquisition) and about where/
how a CDN can acquire the content.
Content: Any form of digital data. One important form of Content
with additional constraints on distribution and delivery is
continuous media (i.e., where there is a timing relationship between
source and sink).
Content Distribution Metadata: The subset of Content Metadata that is
relevant to the distribution of the content. This is the metadata
required by a CDN in order to enable and control content distribution
and delivery by the CDN. In a CDN Interconnection environment, some
of the Content Distribution Metadata may have an intra-CDN scope (and
therefore need not be communicated between CDNs), while some of the
Content Distribution Metadata may have an inter-CDN scope (and
therefore needs to be communicated between CDNs).
Content Distribution Network (CDN) / Content Delivery Network (CDN):
Network infrastructure in which the network elements cooperate at
Layers 4 through 7 for more effective delivery of Content to User
Agents. Typically, a CDN consists of a Request Routing system, a
Distribution system (that includes a set of Surrogates), a Logging
system, and a CDN Control system.
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RFC 6707 CDN Interconnection Problem Statement September 2012
Content Metadata: This is metadata about Content. Content Metadata
comprises:
1. Metadata that is relevant to the distribution of the content (and
therefore relevant to a CDN involved in the delivery of that
content). We refer to this type of metadata as "Content
Distribution Metadata". See also the definition of Content
Distribution Metadata.
2. Metadata that is associated with the actual Content or content
representation, and not directly relevant to the distribution of
that Content. For example, such metadata may include information
pertaining to the Content's genre, cast, rating, etc. as well as
information pertaining to the Content representation's
resolution, aspect ratio, etc.
Content Service: The service offered by a Content Service Provider.
The Content Service encompasses the complete service, which may be
wider than just providing access to items of Content; e.g., the
Content Service also includes any middleware, key distribution,
program guide, etc. that may not require any direct interaction with
the CDN, or CDNs, involved in the distribution and delivery of the
content.
Content Service Provider (CSP): Provides a Content Service to End
Users (which they access via a User Agent). A CSP may own the
Content made available as part of the Content Service, or may license
content rights from another party.
Control system: The function within a CDN responsible for
bootstrapping and controlling the other components of the CDN as well
as for handling interactions with external systems (e.g., handling
delivery service creation/update/removal requests, or specific
service provisioning requests).
Delivery: The function within CDN Surrogates responsible for
delivering a piece of content to the User Agent. For example,
delivery may be based on HTTP progressive download or HTTP adaptive
streaming.
Distribution system: The function within a CDN responsible for
distributing Content Distribution Metadata as well as the Content
itself inside the CDN (e.g., down to the Surrogates).
Downstream CDN: For a given End User request, the CDN (within a pair
of directly interconnected CDNs) to which the request is redirected
by the other CDN (the Upstream CDN). Note that in the case of
successive redirections (e.g., CDN1-->CDN2-->CDN3), a given CDN
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RFC 6707 CDN Interconnection Problem Statement September 2012
(e.g., CDN2) may act as the Downstream CDN for a redirection (e.g.,
CDN1-->CDN2) and as the Upstream CDN for the subsequent redirection
of the same request (e.g., CDN2-->CDN3).
Dynamic CDNI Metadata acquisition: In the context of CDN
Interconnection, dynamic CDNI Metadata acquisition means that a
Downstream CDN acquires CDNI Metadata for content from the Upstream
CDN at some point in time after a request for that content is
delegated to the Downstream CDN by an Upstream CDN (and that specific
CDNI Metadata is not yet available in the Downstream CDN). See also
the definitions for Downstream CDN and Upstream CDN.
Dynamic content acquisition: Dynamic content acquisition is where a
CDN acquires content from the content source in response to an End
User requesting that content from the CDN. In the context of CDN
Interconnection, dynamic acquisition means that a Downstream CDN
acquires the content from content sources (including Upstream CDNs)
at some point in time after a request for that content is delegated
to the Downstream CDN by an Upstream CDN (and that specific content
is not yet available in the Downstream CDN).
End User (EU): The 'real' user of the system, typically a human but
maybe some combination of hardware and/or software emulating a human
(e.g., for automated quality monitoring etc.).
Logging system: The function within a CDN responsible for collecting
the measurement and recording of distribution and delivery
activities. The information recorded by the Logging system may be
used for various purposes, including charging (e.g., of the CSP),
analytics, and monitoring.
Metadata: Metadata in general is data about data.
Network Service Provider (NSP): Provides network-based connectivity/
services to End Users.
Over-the-top (OTT): A service, e.g., content delivery using a CDN,
operated by a different operator than the NSP to which the users of
that service are attached.
Pre-positioned CDNI Metadata acquisition: In the context of CDN
Interconnection, CDNI Metadata pre-positioning is where the
Downstream CDN acquires CDNI Metadata for content prior to, or
independently of, any End User requesting that content from the
Downstream CDN.
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RFC 6707 CDN Interconnection Problem Statement September 2012
Pre-positioned content acquisition: Content pre-positioning is where
a CDN acquires content from the content source prior to, or
independently of, any End User requesting that content from the CDN.
In the context of CDN Interconnection, the Upstream CDN instructs the
Downstream CDN to acquire the content from content sources (including
Upstream CDNs) in advance of, or independently of, any End User
requesting it.
Quality of Experience (QoE): As defined in Section 2.4 of [RFC6390].
Request Routing system: The function within a CDN responsible for
receiving a Content Request from a User Agent, obtaining and
maintaining necessary information about a set of candidate Surrogates
or candidate CDNs, and for selecting and redirecting the user to the
appropriate Surrogate or CDN. To enable CDN Interconnection, the
Request Routing system must also be capable of handling User Agent
Content Requests passed to it by another CDN.
Surrogate: A device/function (often called a cache) that interacts
with other elements of the CDN for the control and distribution of
Content within the CDN and interacts with User Agents for the
delivery of the Content. Typically, Surrogates will cache requested
content so that they can directly deliver the same content in
response to requests from multiple User Agents (and their End Users),
avoiding the need for the content to transit multiple times through
the network core (i.e., from the content origin to the Surrogate).
Upstream CDN: For a given End User request, the CDN (within a pair of
directly interconnected CDNs) that redirects the request to the
other CDN.
User Agent (UA): Software (or a combination of hardware and software)
through which the End User interacts with a Content Service. The
User Agent will communicate with a Content Service for the selection
of content and one or more CDNs for the delivery of the Content.
Such communication is not restricted to HTTP and may be via a variety
of protocols. Examples of User Agents (non-exhaustive) are browsers,
Set Top Boxes (STBs), dedicated content applications (e.g., media
players), etc.
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RFC 6707 CDN Interconnection Problem Statement September 20121.2. CDN Background
Readers are assumed to be familiar with the architecture, features,
and operation of CDNs. For readers less familiar with the operation
of CDNs, the following resources may be useful:
o RFC 3040 [RFC3040] describes many of the component technologies
that are used in the construction of a CDN.
o Taxonomy [TAXONOMY] compares the architecture of a number of CDNs.
o RFC 3466 [RFC3466] and RFC 3570 [RFC3570] are the output of the
IETF Content Distribution Internetworking (CDI) working group,
which was closed in 2003.
Note: Some of the terms used in this document are similar to terms
used in the above referenced documents. When reading this document,
terms should be interpreted as having the definitions provided in
Section 1.1.
2. CDN Interconnection Use Cases
An increasing number of NSPs are deploying CDNs in order to deal
cost-effectively with the growing usage of on-demand video services
and other content delivery applications.
CDNs allow caching of content closer to the edge of a network so that
a given item of content can be delivered by a CDN Surrogate (i.e., a
cache) to multiple User Agents (and their End Users) without
transiting multiple times through the network core (i.e., from the
content origin to the Surrogate). This contributes to bandwidth cost
reductions for the NSP and to improved quality of experience for the
End Users. CDNs also enable replication of popular content across
many Surrogates, which enables content to be served to large numbers
of User Agents concurrently. This also helps in dealing with
situations such as flash crowds and denial-of-service attacks.
The CDNs deployed by NSPs are not just restricted to the delivery of
content to support the Network Service Provider's own 'walled garden'
services, such as IP delivery of television services to Set Top
Boxes, but are also used for delivery of content to other devices,
including PCs, tablets, mobile phones, etc.
Some service providers operate over multiple geographies and federate
multiple affiliate NSPs. These NSPs typically operate independent
CDNs. As they evolve their services (e.g., for seamless support of
content services to nomadic users across affiliate NSPs), there is a
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RFC 6707 CDN Interconnection Problem Statement September 2012
need for interconnection of these CDNs; this represents a first use
case for CDNI. However, there are no open specifications, nor common
best practices, defining how to achieve such CDN Interconnection.
CSPs have a desire to be able to get (some of) their content to very
large numbers of End Users, who are often distributed across a number
of geographies, while maintaining a high quality of experience, all
without having to maintain direct business relationships with many
different CDN Providers (or having to extend their own CDN to a large
number of locations). Some NSPs are considering interconnecting
their respective CDNs (as well as possibly over-the-top CDNs) so that
this collective infrastructure can address the requirements of CSPs
in a cost-effective manner. This represents a second use case for
CDNI. In particular, this would enable the CSPs to benefit from
on-net delivery (i.e., within the Network Service Provider's own
network/CDN footprint) whenever possible and off-net delivery
otherwise, without requiring the CSPs to maintain direct business
relationships with all the CDNs involved in the delivery. Again, CDN
Providers (NSPs or over-the-top CDN operators) are faced with a lack
of open specifications and best practices.
NSPs have often deployed CDNs as specialized cost-reduction projects
within the context of a particular service or environment. Some NSPs
operate separate CDNs for separate services. For example, there may
be a CDN for managed IPTV service delivery, a CDN for web-TV
delivery, and a CDN for video delivery to mobile terminals. As NSPs
integrate their service portfolio, there is a need for
interconnecting these CDNs, representing a third use case for CDNI.
Again, NSPs face the problem of lack of open interfaces for CDN
Interconnection.
For operational reasons (e.g., disaster, flash crowd) or commercial
reasons, an over-the-top CDN may elect to make use of another CDN
(e.g., an NSP CDN with on-net Surrogates for a given footprint) for
serving a subset of the user requests (e.g., requests from users
attached to that NSP), which results in a fourth use case for CDNI
because CDN Providers (over-the-top CDN Providers or NSPs) are faced
with a lack of open specifications and best practices.
Use cases for CDN Interconnection are further discussed in
[CDNI-USE-CASES].
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RFC 6707 CDN Interconnection Problem Statement September 20123. CDN Interconnection Model and Problem Area for IETF
This section discusses the problem area for the IETF work on CDN
Interconnection.
Interconnecting CDNs involves interactions among multiple different
functions and components that form each CDN. Only some of those
require additional specification by the IETF.
Some NSPs have started to perform experiments to explore whether
their CDN use cases can already be addressed with existing CDN
implementations. One set of such experiments is documented in
[CDNI-EXPERIMENTS]. The conclusions of those experiments are that
while some basic limited CDN Interconnection functionality can be
achieved with existing CDN technology, the current lack of any
standardized CDNI interfaces with the necessary level of
functionality such as those discussed in this document is preventing
the deployment of CDN Interconnection.
Listed below are the four interfaces required to interconnect a pair
of CDNs and that constitute the problem space of CDN Interconnection
along with the required functionality of each interface for which
standards do not currently exist. As part of the development of the
CDNI interfaces, it will also be necessary to agree on common
mechanisms for how to identify and name the data objects that are to
be interchanged between interconnected CDNs.
The use of the term "interface" is meant to encompass the protocol
over which CDNI data representations (e.g., CDNI Metadata objects)
are exchanged as well as the specification of the data
representations themselves (i.e., what properties/fields each object
contains, its structure, etc.).
o CDNI Control interface: This interface allows the "CDNI Control"
system in interconnected CDNs to communicate. This interface may
support the following:
* Allow bootstrapping of the other CDNI interfaces (e.g.,
interface address/URL discovery and establishment of security
associations).
* Allow configuration of the other CDNI interfaces (e.g.,
Upstream CDN specifies information to be reported through the
CDNI Logging interface).
* Allow the Downstream CDN to communicate static (or fairly
static) information about its delivery capabilities and
policies.
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RFC 6707 CDN Interconnection Problem Statement September 2012
* Allow bootstrapping of the interface between CDNs for content
acquisition (even if that interface itself is outside the scope
of the CDNI work).
* Allow an Upstream CDN to initiate or request specific actions
to be undertaken in the Downstream CDN. For example, to allow
an Upstream CDN to initiate content or CDNI Metadata
acquisition (pre-positioning) or to request the invalidation
or purging of content files and/or CDNI Metadata in a
Downstream CDN.
o CDNI Request Routing interface: This interface allows the Request
Routing systems in interconnected CDNs to communicate to ensure
that an End User request can be (re)directed from an Upstream CDN
to a Surrogate in the Downstream CDN, in particular where
selection responsibilities may be split across CDNs (for example,
the Upstream CDN may be responsible for selecting the Downstream
CDN, while the Downstream CDN may be responsible for selecting the
actual Surrogate within that Downstream CDN). In particular, the
functions of the CDN Request Routing interface may be divided as
follows:
* A CDNI Request Routing Redirection interface, which allows the
Upstream CDN to query the Downstream CDN at request routing
time before redirecting the request to the Downstream CDN.
* A CDNI Footprint & Capabilities advertisement interface, which
allows the Downstream CDN to provide to the Upstream CDN
(static or dynamic) information (e.g., resources, footprint,
load) to facilitate selection of the Downstream CDN by the
Upstream CDN Request Routing system when processing subsequent
Content Requests from User Agents.
o CDNI Metadata interface: This interface allows the Distribution
system in interconnected CDNs to communicate to ensure that CDNI
Metadata can be exchanged across CDNs. See Section 1.1 for the
definition and examples of CDNI Metadata.
o CDNI Logging interface: This interface allows the Logging system
in interconnected CDNs to communicate the relevant activity logs
in order to allow log-consuming applications to operate in a
multi-CDN environment. For example, an Upstream CDN may collect
delivery logs from a Downstream CDN in order to perform
consolidated charging of the CSP or for settlement purposes across
CDNs. Similarly, an Upstream CDN may collect delivery logs from a
Downstream CDN in order to provide consolidated reporting and
monitoring to the CSP.
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RFC 6707 CDN Interconnection Problem Statement September 2012
Note that the actual grouping of functionalities under these four
interfaces is considered tentative at this stage and may be changed
after further study (e.g., some subset of functionality may be moved
from one interface into another).
The above list covers a significant potential problem space, in part
because in order to interconnect two CDNs there are several 'touch
points' that require standardization. However, it is expected that
the CDNI interfaces need not be defined from scratch and instead can
reuse or leverage existing protocols to a very significant extent;
this is discussed further in Section 4.
The interfaces that form the CDNI problem area are illustrated in
Figure 1.
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RFC 6707 CDN Interconnection Problem Statement September 2012
As illustrated in Figure 1, the acquisition of content between
interconnected CDNs is out of scope for CDNI; this deserves some
additional explanation. The consequence of such a decision is that
the CDNI problem space described in this document is focused on only
defining the control plane for CDNI, and the CDNI data plane (i.e.,
the acquisition and distribution of the actual content objects) is
out of scope. The rationale for such a decision is that CDNs today
typically already use standardized protocols such as HTTP, FTP,
rsync, etc. to acquire content from their CSP customers, and it is
expected that the same protocols could be used for acquisition
between interconnected CDNs. Therefore, the problem of content
acquisition is considered already solved, and all that is required
with respect to content acquisition from specifications developed by
the CDNI working group is to describe within the CDNI Metadata the
parameters to use to retrieve the content -- for example, the IP
address/hostname to connect to, what protocol to use to retrieve the
content, etc.
4. Scoping the CDNI Problem
This section outlines how the scope of work addressing the CDNI
problem space can be constrained through reuse or leveraging of
existing protocols to implement the CDNI interfaces. This discussion
is not intended to preempt any working group decision as to the most
appropriate protocols, technologies, and solutions to select to
realize the CDNI interfaces but is intended as an illustration of the
fact that the CDNI interfaces need not be created in a vacuum and
that reuse or leverage of existing protocols is likely possible.
The four CDNI interfaces (CDNI Control interface, CDNI Request
Routing interface, CDNI Metadata interface, and CDNI Logging
interface) described in Section 3 within the CDNI problem area are
all control plane interfaces operating at the application layer
(Layer 7 in the OSI network model). Firstly, since it is not
expected that these interfaces would exhibit unique session,
transport, or network requirements as compared to the many other
existing applications in the Internet, it is expected that the CDNI
interfaces will be defined on top of existing session, transport, and
network protocols.
Secondly, although a new application protocol could be designed
specifically for CDNI, our analysis below shows that this is
unnecessary, and it is recommended that existing application
protocols be reused or leveraged (HTTP [RFC2616], the Atom Publishing
Protocol [RFC5023], the Extensible Messaging and Presence Protocol
(XMPP) [RFC6120], for example) to realize the CDNI interfaces.
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RFC 6707 CDN Interconnection Problem Statement September 20124.1. CDNI Control Interface
The CDNI Control interface allows the Control system in
interconnected CDNs to communicate. The exact inter-CDN control
functionality required to be supported by the CDNI Control interface
is less well defined than the other three CDNI interfaces at this
time.
It is expected that for the Control interface, as for the other CDNI
interfaces, existing protocols can be reused or leveraged.
4.2. CDNI Request Routing Interface
The CDNI Request Routing interface enables a Request Routing function
in an Upstream CDN to query a Request Routing function in a
Downstream CDN to determine if the Downstream CDN is able (and
willing) to accept the delegated Content Request. It also allows the
Downstream CDN to control what should be returned to the User Agent
in the redirection message by the upstream Request Routing function.
The CDNI Request Routing interface is therefore a fairly
straightforward request/response interface and could be implemented
over any number of request/response protocols. For example, it may
be implemented as a WebService using one of the common WebServices
methodologies (Extensible Markup Language-Remote Procedure Calling
(XML-RPC), HTTP query to a known URI, etc.). This removes the need
for the CDNI working group to define a new protocol for the request/
response element of the CDNI Request Routing interface.
Additionally, as discussed in Section 3, the CDNI Request Routing
interface is also expected to enable a Downstream CDN to provide to
the Upstream CDN (static or dynamic) information (e.g., resources,
footprint, load) to facilitate selection of the Downstream CDN by the
Upstream CDN Request Routing system when processing subsequent
Content Requests from User Agents. It is expected that such
functionality of the CDNI request routing could be specified by the
CDNI working group with significant leveraging of existing IETF
protocols supporting the dynamic distribution of reachability
information (for example, by leveraging existing routing protocols)
or supporting application-level queries for topological information
(for example, by leveraging Application-Layer Traffic Optimization
(ALTO) [RFC5693]).
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RFC 6707 CDN Interconnection Problem Statement September 20124.3. CDNI Metadata Interface
The CDNI Metadata interface enables the Distribution system in a
Downstream CDN to request CDNI Metadata from an Upstream CDN so that
the Downstream CDN can properly process and respond to redirection
requests received over the CDNI Request Routing interface and Content
Requests received directly from User Agents.
The CDNI Metadata interface is therefore similar to the CDNI Request
Routing interface because it is a request/response interface with the
potential addition that CDNI Metadata search may have more complex
semantics than a straightforward Request Routing redirection request.
Therefore, like the CDNI Request Routing interface, the CDNI Metadata
interface may be implemented as a WebService using one of the common
WebServices methodologies (XML-RPC, HTTP query to a known URI, etc.)
or possibly using other existing protocols such as XMPP [RFC6120].
This removes the need for the CDNI working group to define a new
protocol for the request/response element of the CDNI Metadata
interface.
4.4. CDNI Logging Interface
The CDNI Logging interface enables details of content distribution
and delivery activities to be exchanged between interconnected CDNs
-- for example, the exchange of log records related to the delivery
of content, similar to the log records recorded in a web server's
access log.
Several protocols already exist that could potentially be used to
exchange CDNI logs between interconnected CDNs, including the Simple
Network Management Protocol (SNMP), syslog, FTP (and secure
variants), HTTP POST, etc.
5. Security Considerations
Distribution of content by a CDN comes with a range of security
considerations, such as how to enforce control of access to the
content by End Users in line with the CSP policy, or how to trust the
logging information generated by the CDN for the purposes of charging
the CSP. These security aspects are already dealt with by CDN
Providers and CSPs today in the context of standalone CDNs. However,
interconnection of CDNs introduces a new set of security
considerations by extending the trust model to a chain of trust
(i.e., the CSP "trusts" a CDN that "trusts" another CDN). The
mechanisms used to mitigate these risks in multi-CDN environments may
be similar to those used in the single-CDN case, but their
suitability in this more complex environment must be validated.
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The interconnection of CDNs may also introduce additional privacy
considerations on top of those that apply to the single-CDN case. In
a multi-CDN environment, the different CDNs may reside in different
legal regimes that require differing privacy requirements to be
enforced. Such privacy requirements may impact the granularity of
information that can be exchanged across the CDNI interfaces. For
example, the Logging system in a Downstream CDN may need to apply
some degree of anonymization, obfuscation, or even the complete
removal of some fields before exchanging log records containing
details of End User deliveries with an Upstream CDN.
Maintaining the security of the content itself, its associated
metadata (including delivery policies), and the CDNs distributing and
delivering it, are critical requirements for both CDN Providers and
CSPs, and the CDN Interconnection interfaces must provide sufficient
mechanisms to maintain the security of the overall system of
interconnected CDNs as well as the information (content, metadata,
logs, etc.) distributed and delivered through any set of
interconnected CDNs.
5.1. Security of the CDNI Control Interface
Information exchanged between interconnected CDNs over this interface
is of a sensitive nature. A pair of CDNs use this interface to allow
bootstrapping of all the other CDNI interfaces, possibly including
establishment of the mechanisms for securing these interfaces.
Therefore, corruption of that interface may result in corruption of
all other interfaces. Using this interface, an Upstream CDN may
pre-position or delete content or metadata in a Downstream CDN, a
Downstream CDN may provide administrative information to an Upstream
CDN, etc. All of these operations require that the peer CDNs are
appropriately authenticated and that the confidentiality and
integrity of information flowing between them can be ensured.
5.2. Security of the CDNI Request Routing Interface
Appropriate levels of authentication and confidentiality must be used
in this interface because it allows an Upstream CDN to query the
Downstream CDN in order to redirect requests, and conversely, allows
the Downstream CDN to influence the Upstream CDN's Request Routing
function.
In the absence of appropriate security on this interface, a rogue
Upstream CDN could inundate Downstream CDNs with bogus requests or
have the Downstream CDN send the rogue Upstream CDN private
information. Also, a rogue Downstream CDN could influence the
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Upstream CDN so the Upstream CDN redirects requests to the rogue
Downstream CDN or another Downstream CDN in order to, for example,
attract additional delivery revenue.
5.3. Security of the CDNI Metadata Interface
This interface allows a Downstream CDN to request CDNI Metadata from
an Upstream CDN, and therefore the Upstream CDN must ensure that the
former is appropriately authenticated before sending the data.
Conversely, a Downstream CDN must authenticate an Upstream CDN before
requesting metadata to insulate itself from poisoning by rogue
Upstream CDNs. The confidentiality and integrity of the information
exchanged between the peers must be protected.
5.4. Security of the CDNI Logging Interface
Logging data consists of potentially sensitive information (which End
User accessed which media resource, IP addresses of End Users,
potential names and subscriber account information, etc.).
Confidentiality of this information must be protected as log records
are moved between CDNs. This information may also be sensitive from
the viewpoint that it can be the basis for charging across CDNs.
Therefore, appropriate levels of protection are needed against
corruption, duplication, and loss of this information.
6. Acknowledgements
The authors would like to thank Andre Beck, Gilles Bertrand, Mark
Carlson, Bruce Davie, David Ferguson, Yiu Lee, Kent Leung, Will Li,
Kevin Ma, Julien Maisonneuve, Guy Meador, Larry Peterson, Emile
Stephan, Oskar van Deventer, Mahesh Viveganandhan, and Richard Woundy
for their review comments and contributions to the text.
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RFC 6707 CDN Interconnection Problem Statement September 2012Appendix A. Design Considerations for Realizing the CDNI Interfaces
This section expands on how CDNI interfaces can reuse and leverage
existing protocols before describing each CDNI interface individually
and highlighting example candidate protocols that could be considered
for reuse or leveraging to implement the CDNI interfaces. However,
the options discussed here are purely examples and do not present any
consensus on protocols to be used later on.
A.1. CDNI Control Interface
The CDNI Control interface allows the Control system in
interconnected CDNs to communicate. The exact inter-CDN control
functionality required to be supported by the CDNI Control interface
is less well defined than the other three CDNI interfaces at this
time.
However, as discussed in Section 3, the CDNI Control interface may be
required to support functionality similar to the following:
o Allow an Upstream CDN and Downstream CDN to establish, update, or
terminate their CDNI interconnection.
o Allow bootstrapping of the other CDNI interfaces (e.g., protocol
address discovery and establishment of security associations).
o Allow configuration of the other CDNI interfaces (e.g., Upstream
CDN specifies information to be reported through the CDNI Logging
interface).
o Allow the Downstream CDN to communicate static information about
its delivery capabilities, resources, and policies.
o Allow bootstrapping of the interface between CDNs for content
acquisition (even if that interface itself is outside the scope of
the CDNI work).
It is expected that for the Control interface, as for the other CDNI
interfaces, existing protocols can be reused or leveraged. Those
will be considered once the requirements for the Control interface
have been refined.
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RFC 6707 CDN Interconnection Problem Statement September 2012A.2. CDNI Request Routing Interface
The CDNI Request Routing interface enables a Request Routing function
in an Upstream CDN to query a Request Routing function in a
Downstream CDN to determine if the Downstream CDN is able (and
willing) to accept the delegated Content Request and to allow the
Downstream CDN to control what the upstream Request Routing function
should return to the User Agent in the redirection message.
Therefore, the CDNI Request Routing interface needs to offer a
mechanism for an Upstream CDN to issue a "Redirection Request" to a
Downstream CDN. The Request Routing interface needs to be able to
support scenarios where the initial User Agent request to the
Upstream CDN is received over DNS as well as over a content-specific
application protocol (e.g., HTTP, the Real Time Streaming Protocol
(RTSP), the Real Time Messaging Protocol (RTMP), etc.).
Therefore, a Redirection Request is expected to contain information
such as:
o The protocol (e.g., DNS, HTTP) over which the Upstream CDN
received the initial User Agent request.
o Additional details of the User Agent request that are required to
perform effective Request Routing by the Downstream CDN. For DNS,
this would typically be the IP address of the DNS resolver making
the request on behalf of the User Agent. For requests received
over content-specific application protocols, the Redirection
Request could contain significantly more information related to
the original User Agent request but at a minimum is expected to
include the User Agent's IP address, the equivalent of the HTTP
Host header, and the equivalent of the HTTP abs_path as defined in
[RFC2616].
It should be noted that the CDNI architecture needs to consider that
a Downstream CDN may receive requests from User Agents without first
receiving a Redirection Request from an Upstream CDN for the
corresponding User Agent request because, for example:
o User Agents (or DNS resolvers) may cache DNS or application
responses from Request Routers.
o Responses to Redirection Requests over the Request Routing
interface may be cacheable.
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o Some CDNs may rely on simple coarse policies, e.g., CDN B agrees
to always serve CDN A's delegated redirection requests, in which
case the necessary redirection details are exchanged out of band
(of the CDNI interfaces), e.g., configured.
On receiving a Redirection Request, the Downstream CDN will use the
information provided in the request to determine if it is able (and
willing) to accept the delegated Content Request and needs to return
the result of its decision to the Upstream CDN.
Thus, a Redirection Response from the Downstream CDN is expected to
contain information such as:
o Status code indicating acceptance or rejection (possibly with
accompanying reasons).
o Information to allow redirection by the Upstream CDN. In the case
of DNS-based request routing, this is expected to include the
equivalent of a DNS record(s) (e.g., a CNAME) that the Upstream
CDN should return to the requesting DNS resolver. In the case of
application-based request routing, this is expected to include the
information necessary to construct the application-specific
redirection response(s) to return to the requesting User Agent.
For HTTP requests from User Agents, this could include a URI that
the Upstream CDN could return in an HTTP 3xx response.
The CDNI Request Routing interface is therefore a fairly
straightforward request/response interface and could be implemented
over any number of request/response protocols. For example, it may
be implemented as a WebService using one of the common WebServices
methodologies (XML-RPC, HTTP query to a known URI, etc.). This
removes the need for the CDNI working group to define a new protocol
for the request/response element of the CDNI Request Routing
interface. Thus, the CDNI working group would be left only with the
task of specifying:
o The recommended request/response protocol to use along with any
additional semantics and procedures that are specific to the CDNI
Request Routing interface (e.g., handling of malformed requests/
responses).
o The syntax (i.e., representation/encoding) of the redirection
requests and responses.
o The semantics (i.e., meaning and expected contents) of the
redirection requests and responses.
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Additionally, as discussed in Section 3, the CDNI Request Routing
interface is also expected to enable a Downstream CDN to provide to
the Upstream CDN (static or dynamic) information (e.g., resources,
footprint, load) to facilitate selection of the Downstream CDN by the
Upstream CDN Request Routing system when processing subsequent
Content Requests from User Agents. It is expected that such
functionality of the CDNI request routing could be specified by the
CDNI working group with significant leveraging of existing IETF
protocols supporting the dynamic distribution of reachability
information (for example, by leveraging existing routing protocols)
or supporting application-level queries for topological information
(for example, by leveraging ALTO).
A.3. CDNI Metadata Interface
The CDNI Metadata interface enables the Distribution system in a
Downstream CDN to obtain CDNI Metadata from an Upstream CDN so that
the Downstream CDN can properly process and respond to:
o Redirection Requests received over the CDNI Request Routing
interface.
o Content Requests received directly from User Agents.
The CDNI Metadata interface needs to offer a mechanism for an
Upstream CDN to:
o Distribute/update/remove CDNI Metadata to a Downstream CDN.
and/or to allow a Downstream CDN to:
o Make direct requests for CDNI Metadata objects.
o Make recursive requests for CDNI Metadata -- for example, to
enable a Downstream CDN to walk down a tree of objects with
inter-object relationships.
The CDNI Metadata interface is therefore similar to the CDNI Request
Routing interface because it is a request/response interface with the
potential addition that CDNI Metadata search may have more complex
semantics than a straightforward Request Routing redirection request.
Therefore, like the CDNI Request Routing interface, the CDNI Metadata
interface may be implemented as a WebService using one of the common
WebServices methodologies (XML-RPC, HTTP query to a known URI, etc.)
or possibly using other existing protocols such as XMPP [RFC6120].
This removes the need for the CDNI working group to define a new
protocol for the request/response element of the CDNI Metadata
interface.
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Thus, the CDNI working group would be left only with the task of
specifying:
o The recommended request/response protocol to use along with any
additional semantics that are specific to the CDNI Metadata
interface (e.g., handling of malformed requests/responses).
o The syntax (i.e., representation/encoding) of the CDNI Metadata
objects that will be exchanged over the interface.
o The semantics (i.e., meaning and expected contents) of the
individual properties of a Metadata object.
o How the relationships between different CDNI Metadata objects are
represented.
A.4. CDNI Logging Interface
The CDNI Logging interface enables details of content distribution
and delivery activities to be exchanged between interconnected CDNs,
such as log records related to the delivery of content (similar to
the log records recorded in a web server's access log).
Within CDNs today, log records are used for a variety of purposes.
Specifically, CDNs use logs to generate Call Data Records (CDRs) for
passing to billing and payment systems and to real-time (and near
real-time) analytics systems. Such applications place requirements
on the CDNI Logging interface to support guaranteed and timely
delivery of log messages between interconnected CDNs. It may also be
necessary to be able to prove the integrity of received log messages.
Several protocols already exist that could potentially be used to
exchange CDNI logs between interconnected CDNs, including SNMP traps,
syslog, FTP, HTTP POST, etc., although it is likely that some of the
candidate protocols may not be well suited to meet all the
requirements of CDNI. For example, SNMP traps pose scalability
concerns, and SNMP does not support guaranteed delivery of traps and
therefore could result in log records being lost and the consequent
CDRs and billing records for that content delivery not being
produced, as well as that content delivery being invisible to any
analytics platforms.
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RFC 6707 CDN Interconnection Problem Statement September 2012
Although it is not necessary to define a new protocol for exchanging
logs across the CDNI Logging interface, the CDNI working group would
still need to specify:
o The recommended protocol to use.
o A default set of log fields and of their syntax and semantics.
Today there is no standard set of common log fields across
different content delivery protocols, and in some cases there is
not even a standard set of log field names and values for
different implementations of the same delivery protocol.
o A default set of conditions that trigger log records to be
generated.
Appendix B. Additional Material
This section records related information that was produced as part of
defining the CDNI problem statement.
B.1. Non-Goals for IETF
Listed below are aspects of content delivery that the authors propose
be kept outside of the scope of the CDNI working group:
o The interface between the Content Service Provider and the
Authoritative CDN (i.e., the Upstream CDN contracted by the CSP
for delivery by this CDN or by its Downstream CDNs).
o The delivery interface between the delivering CDN Surrogate and
the User Agent, such as streaming protocols.
o The request interface between the User Agent and the Request
Routing system of a given CDN. Existing IETF protocols (e.g.,
HTTP, RTSP, DNS) are commonly used by User Agents to request
content from a CDN and by CDN Request Routing systems to redirect
the User Agent requests. The CDNI working group need not define
new protocols for this purpose. Note, however, that the CDNI
control plane interface may indirectly affect some of the
information exchanged through the request interface (e.g., URI).
o The content acquisition interface between CDNs (i.e., the data
plane interface for actual delivery of a piece of content from one
CDN to the other). This is expected to use existing protocols
such as HTTP or protocols defined in other forums for content
acquisition between an origin server and a CDN (e.g., HTTP-based
C2 reference point of the Alliance for Telecommunications Industry
Solutions IPTV Interoperability Forum Content on Demand service
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RFC 6707 CDN Interconnection Problem Statement September 2012
(ATIS IIF CoD)). The CDN Interconnection problem space described
in this document may therefore only concern itself with the
agreement/negotiation aspects of which content acquisition
protocol is to be used between two interconnected CDNs in view of
facilitating interoperability.
o End User/User Agent Authentication. End User/User Agent
authentication and authorization are the responsibility of the
Content Service Provider.
o Content preparation, including encoding and transcoding. The CDNI
architecture aims at allowing distribution across interconnected
CDNs of content treated as opaque objects. Interpretation and
processing of the objects, as well as optimized delivery of these
objects by the Surrogate to the End User, are outside the scope of
CDNI.
o Digital Rights Management (DRM). DRM is an end-to-end issue
between a content protection system and the User Agent.
o Applications consuming CDNI logs (e.g., charging, analytics,
reporting, ...).
o Internal CDN interfaces and protocols (i.e., interfaces and
protocols within one CDN).
o Scalability of individual CDNs. While scalability of the CDNI
interfaces/approach is in scope, how an individual CDN scales is
out of scope.
o Actual algorithms for selection of CDNs or Surrogates by Request
Routing systems (however, some specific parameters required as
input to these algorithms may be in scope when they need to be
communicated across CDNs).
o Surrogate algorithms. For example, caching algorithms and content
acquisition methods are outside the scope of the CDNI work.
Content management (e.g., Content Deletion) as it relates to CDNI
content management policies is in scope, but the internal
algorithms used by a cache to determine when to no longer cache an
item of Content (in the absence of any specific metadata to the
contrary) is out of scope.
o Element management interfaces.
o Commercial, business, and legal aspects related to the
interconnections of CDNs.
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RFC 6707 CDN Interconnection Problem Statement September 2012B.2. Relationship to Relevant IETF Working Groups and IRTF Research
Groups
B.2.1. ALTO WG
As stated in the ALTO working group charter [ALTO-Charter]:
The Working Group will design and specify an Application-Layer
Traffic Optimization (ALTO) service that will provide applications
with information to perform better-than-random initial peer
selection. ALTO services may take different approaches at
balancing factors such as maximum bandwidth, minimum cross-domain
traffic, lowest cost to the user, etc. The working group will
consider the needs of BitTorrent, tracker-less P2P, and other
applications, such as content delivery networks (CDN) and mirror
selection.
In particular, the ALTO service can be used by a CDN Request Routing
system to improve its selection of a CDN Surrogate to serve a
particular User Agent request (or to serve a request from another
Surrogate). [ALTO-CDN-USE-CASES] describes a number of use cases for
a CDN to be able to obtain network topology and cost information from
an ALTO server(s) and discusses how CDN Request Routing could be used
as an integration point of ALTO into CDNs. It is possible that the
ALTO service could be used in the same manner in a multi-CDN
environment based on CDN Interconnection. For example, an Upstream
CDN may take advantage of the ALTO service in its decision for
selecting a Downstream CDN to which a user request should be
delegated.
However, the current work of ALTO is complementary to and does not
overlap with the work described in this document because the
integration between ALTO and a CDN is an internal decision for a
specific CDN and is therefore out of scope for the CDNI working
group. One area for further study is whether additional information
should be provided by an ALTO service to facilitate CDNI CDN
selection.
B.2.2. DECADE WG
The DECADE working group [DECADE-Charter] is addressing the problem
of reducing traffic on the last-mile uplink, as well as backbone and
transit links caused by peer-to-peer (P2P) streaming and file-sharing
applications. It addresses the problem by enabling an application
endpoint to make content available from an in-network storage service
and by enabling other application endpoints to retrieve the content
from there.
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Exchanging data through the in-network storage service in this
manner, instead of through direct communication, provides significant
gain where:
o The network capacity/bandwidth from the in-network storage service
to the application endpoint significantly exceeds the capacity/
bandwidth from application endpoint to application endpoint (e.g.,
because of an end-user uplink bottleneck); and
o The content is to be accessed by multiple instances of application
endpoints (e.g., as is typically the case for P2P applications).
While, as is the case for any other data distribution application,
the DECADE architecture and mechanisms could potentially be used for
exchange of CDNI control plane information via an in-network storage
service (as opposed to directly between the entities terminating the
CDNI interfaces in the neighbor CDNs), we observe that:
o CDNI would operate as a "Content Distribution Application" from
the DECADE viewpoint (i.e., would operate on top of DECADE).
o There do not seem to be obvious benefits in integrating the DECADE
control plane responsible for signaling information relating to
control of the in-network storage service itself, and the CDNI
control plane responsible for application-specific CDNI
interactions (such as exchange of CDNI Metadata, CDNI request
redirection, and transfer of CDNI logging information).
o There would typically be limited benefits in making use of a
DECADE in-network storage service because the CDNI interfaces are
expected to be terminated by a very small number of CDNI clients
(if not one) in each CDN, and the CDNI clients are expected to
benefit from high bandwidth/capacity when communicating directly
to each other (at least as high as if they were communicating via
an in-network storage server).
The DECADE in-network storage architecture and mechanisms may
theoretically be used for the acquisition of the content objects
themselves between interconnected CDNs. It is not expected that this
would have obvious benefits in typical situations where a content
object is acquired only once from an Upstream CDN to a Downstream CDN
(and then distributed as needed inside the Downstream CDN). But it
might have benefits in some particular situations. Since the
acquisition protocol between CDNs is outside the scope of the CDNI
work, this question is left for further study.
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RFC 6707 CDN Interconnection Problem Statement September 2012
The DECADE in-network storage architecture and mechanisms may
potentially also be used within a given CDN for the distribution of
the content objects themselves among Surrogates of that CDN. Since
the CDNI work does not concern itself with operation within a CDN,
this question is left for further study.
Therefore, the work of DECADE may be complementary to, but does not
overlap with, the CDNI work described in this document.
B.2.3. PPSP WG
As stated in the PPSP working group charter [PPSP-Charter]:
The Peer-to-Peer Streaming Protocol (PPSP) working group develops
two signaling and control protocols for a peer-to-peer (P2P)
streaming system for transmitting live and time-shifted media
content with near real-time delivery requirements...
... The PPSP working group designs a protocol for signaling and
control between trackers and peers (the PPSP "tracker protocol")
and a signaling and control protocol for communication among the
peers (the PPSP "peer protocol"). The two protocols enable peers
to receive streaming data within the time constraints required by
specific content items.
Therefore, PPSP is concerned with the distribution of the streamed
content itself along with the necessary signaling and control
required to distribute the content. As such, it could potentially be
used for the acquisition of streamed content across interconnected
CDNs. But since the acquisition protocol is outside the scope of the
work proposed for CDNI, we leave this for further study. Also,
because of its streaming nature, PPSP is not seen as applicable to
the distribution and control of the CDNI control plane and CDNI data
representations.
Therefore, the work of PPSP may be complementary to, but does not
overlap with, the work described in this document for CDNI.
B.2.4. IRTF P2P Research Group
Some information on CDN Interconnection motivations and technical
issues were presented in the P2P research group at IETF 77. The
presentation can be found in [P2PRG-CDNI].
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